This application is based on Japanese Patent Application No. 9-310205, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a method of effecting color matching between an input color data batch inputted to a color image input device and an output color data batch outputted from a color image output device, or between output color data batches outputted from a plurality of color image output devices which are operated according to the same input color data batch.
2. Discussion of the Related Art
There are known various color image output devices such as a CRT (cathode ray tube) display, a color printer and a color copier, which are adapted to reproduce a color image according to an image data batch (internal color data) received from a color image input device such as a color scanner, a digital camera or a digital video camera, which is adapted to read or obtain an original color image (external color data as viewed by an observer). The image data batch or external color data batch consists of a multiplicity of pixel color data sets representative of colors at respective picture elements which collectively define a color image. The original color image (external color data) is reproduced based on those pixel color data sets.
A color image output device as indicated above is capable of reproducing various colors by mixing three pigments having respective three colors consisting of yellow (Y), magenta (M) and cyan (C), or four pigments having respective four colors consisting of yellow, magenta, cyan and black (K), or alternatively by mixing three colors of light of phosphors consisting of red (R), green (G) and blue (B). The pixel color data sets indicated above are generally expressed by a polychromatic system using three or more actually existing primary colors (such as R, G and B, or Y, M, C and K), for instance, by the trichromatic RGB color mixing system. In the RGB color mixing system, color data sets R, G, B consist of a R value, a G value and a B value which represent concentrations of red, green and blue filters. The color image output device uses the R, G and B values for specifying mixing proportions of the three primary colors, to reproduce a color image on a display screen or a recording medium.
It is desired that the colors of the color image as reproduced by the color image output device be consistent or matched with the colors of the original color image whose data batch is outputted from the color image input device and inputted into the image output device, or with the colors of the color images as reproduced by other color image output devices. Generally, however, the color image input and output devices are adjusted or calibrated independently of each other, in terms of the relationship between the color of the original image and the color represented by the color data sets outputted from the color image input device, and the relationship between the color as represented by the color data sets and the color of the color image as reproduced by the color image output device.
On the other hand, the color of an object can be defined by the tristimulus values X, Y and Z in the XYZ chromaticity coordinate system according to the CIE (Commission Internationale de l""Eclairage), based on a relative spectral distribution obtained by a spectrophotometer which photometrically measures the object. Therefore, it is possible to make color matching adjustments of the color image input and output devices, by actually obtaining a predetermined number of measurements of the tristimulus values X, Y, Z in the XYZ chromatic system, and obtaining relationships between these tristimulus values X, Y, Z (external color data) and color data sets r, g, b (internal color data) used by the color output devices. The XYZ chromatic system may be replaced by other chromatic systems such as a (L*, a*, b*) system.
The color image input and output devices employs either a subtractive mixing of colors or an additive mixing of colors, so that the relationships between the tristimulus values X, Y, Z and the color values r, g, b of the color data sets are non-linear. The non-linear relationships make it difficult to obtain functions or inverse functions representative of higher-resolution relationships by interpolating the predetermined number of measurements of the tristimulus values X, Y, Z. It is possible to obtain relationships between the tristimulus values X, Y, Z and the color data values r, g, b, for a large number of combinations of three colors, for instance, 2563 combinations. However, this requires a tremendously large number of arithmetic operations to deal with all the combinations and a data memory having an accordingly large storage capacity, and is not actually feasible.
It is therefore an object of the present invention to provide a method of effecting color matching, with comparatively easy processing of a relatively small volume of existing color data, without requiring a large number of processing operations.
The above object may be achieved according to the principle of the present invention, which provides a method of effecting color matching between external color data of a first image processing device and external color data of a second image processing device connected to the first image processing device, the external color data representing a color image as viewed by an observer and being distinguished from internal color data which represent a color image to be reproduced, the method comprising: (i) a first interpolating and inverse-estimating step of (a) applying to the first image processing device, as a first number of input color data sets, one of a first batch of external color data and a first batch of internal color data of the first image processing device, to obtain a relationship between the first number of input color data sets and a first number of output color data sets which correspond to the first number of the input color data sets and which constitute the other of the first batch of external color data and the first batch of internal color data, (b) generating a second number of output color data sets by interpolation of the first number of output color data sets, the second number being larger than the first number, and (c) effecting an inverse estimation to obtain a second number of input color data sets corresponding to the second number of output color data sets, to obtain a first high-resolution input-output relationship between the second number of input color data sets and the second number of output color data sets which respectively constitute one and the other of a second batch of external color data and a second batch of internal color data; (ii) a second interpolating and inverse-estimating step of (a) applying to the second image processing device, as a first number of input color data sets of the second image processing device, one of a first batch of external color data and a first batch of internal color data of the second image processing device, to obtain a relationship between the first number of input color data sets of the second image processing device and a first number of output color data sets which correspond to the first number of the input color data of the second image processing device and which constitute the other of the first batch of external color data and the first batch of internal color data of the second image processing device, (b) generating a second number of output color data sets of the second image processing device, by interpolation of the first number of output color data sets of the second image processing device, the second number of the output color data sets of the second image processing device being larger than the first number of output color data sets of the second image processing device, and (c) effecting an inverse estimation to obtain a second number of input color data sets corresponding to the second number of output color data sets of the second image processing device, to obtain a second high-resolution input-output relationship between the second number of input color data sets and the second number of output color data sets of the second image processing device which respectively constitute one and the other of a second batch of external color data and a second batch of internal color data of the second image processing device; and (iii) a color data correlation generating step of generating a color data correlationship for converting the first batch of internal color data of the first image processing device into the second batch of internal color data of the second image processing device, according to the first and second high-resolution input-output relationships.
In the method of the present invention described above, the first interpolating and inverse-estimating step is performed to apply to the first image processing device, as a first number of input color data sets, one of a first batch of external color data and a first batch of internal color data of the first image processing device, to obtain a relationship between the first number of input color data sets and a first number of output color data sets which correspond to the first number of the input color data sets and which constitute the other of the first batch of external color data and the first batch of internal color data. Further, a second number of output color data sets are generated by interpolation of the first number of output color data sets. This second number is larger than the first number. Then, an inverse estimation is effected to obtain a second number of input color data sets corresponding to the second number of output color data sets, to obtain a first high-resolution input-output relationship between the second number of input color data sets and the second number of output color data sets, which respectively constitute one and the other of a second batch of external color data and a second batch of internal color data of the first image processing device. The second interpolating and inverse-estimating step is performed to apply to the second image processing device, as a first number of input color data sets of the second image processing device, one of a first batch of external color data and a first batch of internal color data of the second image processing device, to obtain a relationship between the first number of input color data sets of the second image processing device and a first number of output color data sets which correspond to the first number of the input color data of the second image processing device and which constitute the other of the first batch of external color data and the first batch of internal color data of the second image processing device. Further, a second number of output color data sets of the second image processing device is generated by interpolation of the first number of output color data sets of the second image processing device. Then, an inverse estimation is effected to obtain a second number of input color data sets corresponding to the second number of output color data sets of the second image processing device, to obtain a second high-resolution input-output relationship between the second number of input color data sets and the second number of output color data sets of the second image processing device, which respectively constitute one and the other of a second batch of external color data and a second batch of internal color data of the second image processing device.
Then, the color data correlation generating step is performed to generate a color data correlationship for converting the first batch of internal color data of the first image processing device into the second batch of internal color data of the second image processing device, according to the first and second high-resolution input-output relationships, so that the external color data of the first image processing device and the external color data of the second image processing device can be matched with each other.
In a first preferred form of this invention, the method of the present invention further comprises a data converting step for converting the first batch of internal color data of the first image processing device into the second batch of internal color data of the second image processing device, according to the color data correlationship generated in the color data correlation generating step. In this preferred form of the invention, the conversion of the first batch of internal color data of the first image processing device into the second batch of internal color data of the second image processing device according to the color data correlationship permits color matching between the external color data of the first image processing device and the external color data of the second image processing device.
In a second preferred form of this invention, the first image processing device consists of a color image input device, while the second image processing device consists of a color image output device. In this preferred form of the invention, the first interpolating and inverse-estimating step comprises a step of obtaining a relationship between the first number of input color data sets representative of colors of an original image and the first number of output color data sets generated from the color image input device upon application of the first number of input color data sets thereto, a step of generating the second number of output color data sets by interpolation of the first number of output color data sets, such that the second number of output color data sets have a substantially constant color difference interval, and a step of effecting the inverse estimation to obtain the second number of input color data sets corresponding to the second number of output color data sets, to obtain the first high-resolution input-output relationship of the color image input device. Further, the second interpolating and inverse-estimating step comprises a step of obtaining a relationship between the first number of input color data sets of the color image output device and the first number of output color data sets generated from the color image output device upon application of the first number of input color data sets thereto, a step of generating the second number of output color data sets by interpolation of the first number of output color data sets of the color image output device, such that the second number of output color data sets of the image output device have a substantially constant color difference interval, and a step of effecting the inverse estimation to obtain the second number of input color data sets corresponding to the second number of output color data sets of the color output device, to obtain the second high-resolution input-output relationship of the color image output device. In the above second preferred form of the method of the invention, the color data correlationship generating step comprises a step of generating a color data converting table for converting the second number of the output color data sets of the color image input device into the second number of the input color data sets of the color image output device, according to the first and second high-resolution input-output relationships. In this preferred form of the invention, the conversion of the second number of the output color data sets of the color image input device into the second number of the input color data sets of the color image output device permits color matching between the colors of the original image as represented by the input color data sets of the color image input device and the colors as represented by the output color data sets of the color image output device, that is, the colors of a color image as reproduced by the color image output device according to the input color data sets of the color image output device.
In one advantageous form of the above second preferred form of this invention, the method comprises a data converting step for converting second number of output color data sets of color image input device into second number of input color data sets of color image output device according to color data converting table. The conversion of the output color data sets of the color image input device into the input color data sets of the color image output device according to the color data converting table generated in the color data converting step permits color matching between the colors of the original image as represented by the input color data sets of the color image input device and the colors as represented by the output color data sets of the color image output device, namely, the colors of the color image reproduced by the color image output device.
In a third preferred form of this invention, the first image processing device consists of a first color image output device, while the second image processing device consists of a second color image output device. In this form of the method of the invention, the first interpolating and inverse-estimating step comprises a step of obtaining a relationship between the first number of input color data sets of the first color image output device and the first number of output color data sets generated from the first color image output device upon application of the first number of input color data sets thereto, a step of generating the second number of output color data sets by interpolation of the first number of output color data sets, such that the second number of output color data sets have a substantially constant color difference interval, and a step of effecting the inverse estimation to obtain the second number of input color data sets corresponding to the second number of output color data sets, to obtain the first high-resolution input-output relationship of the first color image output device. Further, the second interpolating and inverse-estimating step comprises a step of obtaining a relationship between the first number of input color data sets of the second color image output device and the first number of output color data sets generated from the second color image output device upon application of the first number of input color data sets thereto, a step of generating the second number of output color data sets by interpolation of the first number of output color data sets of the color image output device, such that the second number of output color data sets of the image output device have a substantially constant color difference interval, and a step of effecting the inverse estimation to obtain the second number of input color data sets corresponding to the second number of output color data sets of the color output device, to obtain the second high-resolution input-output relationship of the second color image output device. The color data correlationship generating step comprises a step of generating a color data converting table for converting the second number of the input color data sets of the first color image output device into the second number of the input color data sets of the second color image output device, according to the first and second high-resolution input-output relationships. The conversion of the input color data sets of the first color image output device into the input color data sets of the second color image output device according to the color data converting table permits color matching between the colors as represented by the output color data sets of the first color image output device and the colors as represented by the output color data sets of the second color image output device, namely, between the color image as reproduced by the first color image output device and the colors of the color image as reproduced by the second color image output device.
In one advantageous arrangement of the above third preferred form of this invention, the method further comprises a data converting step for converting the second number of the input color data sets of the first color image output device into the second number of the input color data sets of the second color image output device according to the color data converting table. This arrangement permits the color matching between the colors of the color image as reproduced by the first color image output device and the colors of the color image as reproduced by the second color image output device.